Experimental superposition of orders of quantum gates

Lorenzo M. Procopio (), Amir Moqanaki, Mateus Araújo, Fabio Costa, Irati Alonso Calafell, Emma G. Dowd, Deny R. Hamel, Lee A. Rozema, Časlav Brukner and Philip Walther ()
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Lorenzo M. Procopio: Faculty of Physics, University of Vienna
Amir Moqanaki: Faculty of Physics, University of Vienna
Mateus Araújo: Faculty of Physics, University of Vienna
Fabio Costa: Faculty of Physics, University of Vienna
Irati Alonso Calafell: Faculty of Physics, University of Vienna
Emma G. Dowd: Faculty of Physics, University of Vienna
Deny R. Hamel: Faculty of Physics, University of Vienna
Lee A. Rozema: Faculty of Physics, University of Vienna
Časlav Brukner: Faculty of Physics, University of Vienna
Philip Walther: Faculty of Physics, University of Vienna

Nature Communications, 2015, vol. 6, issue 1, 1-6

Abstract: Abstract Quantum computers achieve a speed-up by placing quantum bits (qubits) in superpositions of different states. However, it has recently been appreciated that quantum mechanics also allows one to ‘superimpose different operations’. Furthermore, it has been shown that using a qubit to coherently control the gate order allows one to accomplish a task—determining if two gates commute or anti-commute—with fewer gate uses than any known quantum algorithm. Here we experimentally demonstrate this advantage, in a photonic context, using a second qubit to control the order in which two gates are applied to a first qubit. We create the required superposition of gate orders by using additional degrees of freedom of the photons encoding our qubits. The new resource we exploit can be interpreted as a superposition of causal orders, and could allow quantum algorithms to be implemented with an efficiency unlikely to be achieved on a fixed-gate-order quantum computer.

Date: 2015
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DOI: 10.1038/ncomms8913

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